Cinton's decision to appear on Oct. 5 alongside one of his wife's top backers in her campaign for the Democratic presidential nomination comes at a critical time for the San Franciso mayor. Newsom has built a Facebook-Twitter base among younger voters, but Brown, a former two-term governor, has raised more than twice as much money as he has.

Clinton's trip to California next month is expected to include an event in predominantly Latino East Los Angeles as well as a high-dollar fundraiser.

"There has never been a former president who has endorsed a candidate in a primary for statewide office in California," said Art Torres, a former state lawmaker who headed the California Democratic Party from 1995 until earlier this year but is neutral in the current race.

"In 1992, Bill Clinton beat Jerry Brown in California by 7 points. He then beat Bush by 13 points in the general," Torres said. "And then in last year's presidential campaign, Hillary [Clinton] beat Obama by 9 points including a 67-to-32 margin among Latinos."

In addition to helping with fundraising and Latinos, the Newsom campaign is hoping that Clinton's endorsement can help the San Francisco mayor make inroads in Southern California.

While California has never in recent memory seen a former president intervene in a competitive statewide primary, Bill Clinton has already done it in two other 2010 races including Rep. Kendrick Meek's, D-Fla., Florida Senate race and Lt. Gov. Lee Fisher's Ohio Senate race.